From 8a9d6ef5970eea2a42ff9eb537f2a3f5e56aec2c Mon Sep 17 00:00:00 2001
From: Charlie Stanton <charlie@shtanton.xyz>
Date: Wed, 19 Apr 2023 16:54:53 +0100
Subject: Upgrades the substitute command to also act as a filter

Substitute now captures the command after it and only runs it if the substitution is a success
---
 main/command.go | 11 ++++++++---
 main/parse.go   | 13 ++++++++++++-
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/main/command.go b/main/command.go
index ba6f9dc..ea6fb59 100644
--- a/main/command.go
+++ b/main/command.go
@@ -107,6 +107,7 @@ func (cmd DeleteAllCommand) exec(state *ProgramState) {
 
 type SubstituteCommand struct {
 	subex subex.SubexState
+	next Command
 }
 func (cmd SubstituteCommand) exec(state *ProgramState) {
 	valueStream := make(chan walk.WalkValue)
@@ -119,15 +120,19 @@ func (cmd SubstituteCommand) exec(state *ProgramState) {
 	atomStream := walk.Atomise(valueStream)
 	atomsOut, error := subex.RunTransducer(cmd.subex, atomStream)
 	if error {
-		panic("Error running subex")
+		return
 	}
 	valuesOut, err := walk.MemoryCompound(atomsOut)
 	if err != nil {
-		panic("Error compounding atoms")
+		return
 	}
 	state.value = valuesOut
+	cmd.next.exec(state)
 }
 
 type Command interface {
 	exec(*ProgramState)
-}
\ No newline at end of file
+}
+
+type NoopCommand struct {}
+func (cmd NoopCommand) exec(state *ProgramState) {}
\ No newline at end of file
diff --git a/main/parse.go b/main/parse.go
index bf2f0ac..aed87cc 100644
--- a/main/parse.go
+++ b/main/parse.go
@@ -234,7 +234,18 @@ func (p *parser) parseBasicCommand(commandChar rune) Command {
 			return AppendNextCommand{}
 		case 's':
 			subex := p.parseSubex()
-			return SubstituteCommand {subex: subex}
+			var next Command
+			token := p.peek()
+			switch token.typ {
+				case TokenEOF, TokenRBrace:
+					next = NoopCommand{}
+				default:
+					next = p.parseCommand()
+			}
+			return SubstituteCommand {
+				subex: subex,
+				next: next,
+			}
 		case 'i':
 			items := p.parseLiterals()
 			return PrintLiteralsCommand {items: items}
-- 
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